Millennium Chess is played on an 8 by 15 size chess-board, and each player controls nearly twice as many pieces (two queens, two kings, four bishops, etc.). Most of the rules of the game are the same, with some special rules regarding capturing and check-mating the kings. A player's first king can be left in danger of capture and actually captured - once the first king is captured, the remaining king can then be put in check and check-mate as in normal chess.
From the brochure and web-site:
"Millennium Chess was developed to improve the skills of the average to excellent chess player. This is accomplished by maintaining the basic dynamics and structure of the of the game, while increasing the number of strategic moves.
By expanding the size of the battlefield and increasing the number of pieces, a chess player's mind is forced to accept a greater strategic load. This subconsciously builds one's 'chess muscles'."